This invention relates to cardiac pacers, and is specifically directed to an energy conservation system for extending the useful lifetime of an implantable cardiac pacer of the type having an internal primary battery power source.
It has long been recognized that an electronic circuit implanted within the human body may be powered by an external power source, as representatively shown in U.S. Pat. No. 3,727,616. Similarly, many conventional, nonimplanted electronic circuits may be switchably powered either by an internal battery or from an external AC source, as illustrated in British Pat. No. 826,766.
Implantable devices having a plurality of batteries in combination with isolation diodes are shown in U.S. Pat. Nos. 3,738,371 and 3,783,877, while the use of a regulated power supply to power only a portion of a cardiac pacer is shown in U.S. Pat. No. 3,547,127. However, none of the power supply configurations disclosed in the aforementioned references show or suggest an energy conservation system for extending the lifetime of an implantable cardiac pacer having an internal primary battery power source.
Within the past few years, new types of primary batteries have received considerable attention for use in cardiac pacers, primarily because of their potential for enhancing the useful lifetime of such pacers. Principal among the new primary cells employed is the solid-state lithium-iodine chemical cell or lithium battery, which, when used in conjunction with low-current pacer circuitry, can result in a useful battery lifetime in the order of several years. However, the lithium battery possesses a number of features which have heretofore limited its application in cardiac pacers. It exhibits an open-circuit voltage in the order of 2.8 volts, or about twice that of conventional cells, and has an internal resistance which increases throughout the lifetime of the cell. This increasing internal resistance causes a substantial and undesirable drop in output voltage under load. Furthermore, as the lithium battery is a primary cell, it is nonrechargeable, and its energy density is not adequate for constructing small cardiac pacers with useful lifetimes in the range of ten or more years. For these reasons, a substantial need has heretofore existed for energy conservation means for extending the lifetime of an implantable cardiac pacer powered by a primary source such as the lithium battery.